An organic EL device of the simplest structure is generally constituted of a light-emitting layer sandwiched between a pair of counter electrodes and utilizes the following light-emitting phenomenon. Upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer; the energy level after recombination goes back from the conduction band to the valence band with release of energy in the form of light.
In recent years, organic thin films have been used in the development of EL devices. In particular, in an effort to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been focused on commercial application to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer of an aromatic diamine and a light-emitting layer of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, coumarin derivatives and benzophenone derivatives have been investigated as a material for the light-emitting layer, but they merely produced luminance at an extremely low level. Thereafter, europium complexes were tried to utilize the excited triplet state, but failed to emit light at high efficiency.                Patent document 1: JP2003-515897 A        Patent document 2: JP2001-313178 A        Patent document 3: JP2002-305083 A        Patent document 4: JP2002-352957 A        Patent document 5: JPH11-162650 A        Patent document 6: JPH11-176578 A        
A large number of phosphorescent dopants useful for the light-emitting layer of an organic EL device are disclosed in the patent document 1 and elsewhere. A typical example is tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3).
A carbazole compound or CBP described in the patent document 2 is proposed as a host material for use in the light-emitting layer of an organic EL device. Since CBP has a special property of facilitating the flow of holes and obstructing the flow of electrons, the use of CBP as a host material for Ir(ppy)3 that is a green light-emitting phosphorescent material destroys the balanced injection of charges thereby causing excess holes to flow out to the side of the electron-transporting layer and, as a result, the luminous efficiency from Ir(ppy)3 drops.
As a means to solve the aforementioned problems, a hole-blocking layer may be disposed between the light-emitting layer and the electron-transporting layer, for example, in the manner described in the patent document 3. The hole-blocking layer accumulates holes efficiently in the light-emitting layer thereby improving the probability of recombination of holes and electrons in the light-emitting layer and enhancing the luminous efficiency. Examples of the hole-blocking materials currently in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato-N1, O8)aluminum (hereinafter referred to as BAlq). The hole-blocking layer can prevent electrons and holes from recombining in the electron-transporting layer. However, BCP lacks reliability as a hole-blocking material as it tends to crystallize easily even at room temperature and a device comprising BCP shows an extremely short service life. On the other hand, BAlq is reported to have a Tg of approximately 100° C. and help to show a relatively long service life when incorporated in a device; however, BAlq does not have a sufficient hole-blocking ability and the luminous efficiency from Ir(ppy)3 drops.
Now, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as TAZ) described in the patent document 4 is also proposed as a host material for a phosphorescent organic EL device; however, TAZ has a property of facilitating the flow of electrons and obstructing the flow of holes and displaces the light-emitting range toward the side of the hole-transporting layer. Hence, it is conceivable that the luminous efficiency from Ir(ppy)3 may drop depending upon the compatibility of Ir(ppy)3 with the material of choice for the hole-transporting layer. For example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as NPB) is a material most widely used in the hole-transporting layer on account of its excellent performance, high reliability, and long service life; however, it is poorly compatible with Ir(ppy)3 and energy transition occurs from Ir(ppy)3 to NPB to lower the luminous efficiency.
The indolocarbazole compounds disclosed in the patent documents 5 and 6 are recommended for use as hole-transporting materials and their stability is admired. However, these documents do not teach the use as a phosphorescent host material. Moreover, the disclosed compounds differ in structure from those to be provided by this invention.